Mortgages and Other Loans Payable (Details) (USD $)	3 Months Ended	6 Months Ended		6 Months Ended	1 Months Ended									6 Months Ended																																			1 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Series E preferred units	Sep. 30, 2012 Master Repurchase Agreement	Jun. 30, 2013 Master Repurchase Agreement	Dec. 31, 2012 Master Repurchase Agreement	Jun. 30, 2013 Other loan payable due in September, 2019	Dec. 31, 2012 Other loan payable due in September, 2019	Jun. 30, 2013 220 East 42nd Street	Dec. 31, 2012 220 East 42nd Street	Jun. 30, 2013 609 Partners, LLC	Dec. 31, 2012 609 Partners, LLC	Jun. 30, 2013 609 Partners, LLC Series E preferred units	Jun. 30, 2013 125 Park Avenue	Dec. 31, 2012 125 Park Avenue	Jun. 30, 2013 711 Third Avenue	Dec. 31, 2012 711 Third Avenue	Jun. 30, 2013 625 Madison Avenue	Dec. 31, 2012 625 Madison Avenue	Jun. 30, 2013 500 West Putnam	Dec. 31, 2012 500 West Putnam	Jun. 30, 2013 420 Lexington Avenue	Dec. 31, 2012 420 Lexington Avenue	Jun. 30, 2013 Landmark Square	Dec. 31, 2012 Landmark Square	Jun. 30, 2013 485 Lexington Avenue	Dec. 31, 2012 485 Lexington Avenue	Jun. 30, 2013 120 West 45th Street	Dec. 31, 2012 120 West 45th Street	Jun. 30, 2013 300 Main Street	Dec. 31, 2012 300 Main Street	Jun. 30, 2013 762 Madison Avenue	Dec. 31, 2012 762 Madison Avenue	Jun. 30, 2013 2 Herald Square	Dec. 31, 2012 2 Herald Square	Jun. 30, 2013 885 Third Avenue	Dec. 31, 2012 885 Third Avenue	Jun. 30, 2013 One Madison Avenue	Dec. 31, 2012 One Madison Avenue	Jun. 30, 2013 100 Church	Dec. 31, 2012 100 Church	Jun. 30, 2013 919 Third Avenue	Dec. 31, 2012 919 Third Avenue	Jun. 30, 2013 400 East 57th Street	Dec. 31, 2012 400 East 57th Street	Jun. 30, 2013 400 East 58th Street	Dec. 31, 2012 400 East 58th Street	Feb. 28, 2013 1515 Broadway	Apr. 30, 2012 1515 Broadway	Jun. 30, 2013 1515 Broadway	Dec. 31, 2012 1515 Broadway	Jun. 30, 2013 16 Court Street	Jun. 30, 2013 180 Maiden Lane	Dec. 31, 2012 180 Maiden Lane	Jun. 30, 2013 248-252 Bedford Avenue
Mortgages and Other Loans Payable
Interest rate, fixed rate debt (as a percent)								8.00%		5.25%		5.00%			5.75%		4.99%		7.22%		5.52%		7.50%		4.00%		5.61%		6.12%		5.75%		3.75%		5.36%		6.26%		5.91%		4.68%		5.12%		4.13%		4.13%		3.93%		3.93%
Interest rate, floating rate debt (as a percent)						3.20%																																															3.44%	2.40%		2.45%
Total fixed rate debt	$ 4,340,978,000	$ 4,340,978,000	$ 3,457,769,000					$ 50,000,000	$ 50,000,000	$ 183,527,000	$ 185,906,000	$ 23,000	$ 23,000		$ 146,250,000	$ 146,250,000	$ 120,000,000	$ 120,000,000	$ 123,340,000	$ 125,603,000	$ 23,799,000	$ 24,060,000	$ 184,033,000	$ 184,992,000	$ 83,705,000	$ 84,486,000	$ 450,000,000	$ 450,000,000	$ 170,000,000	$ 170,000,000	$ 11,500,000	$ 11,500,000	$ 8,291,000	$ 8,371,000	$ 191,250,000	$ 191,250,000	$ 267,650,000	$ 267,650,000	$ 597,610,000	$ 607,678,000	$ 230,000,000	$ 230,000,000	$ 500,000,000	$ 500,000,000	$ 70,000,000	$ 70,000,000	$ 30,000,000	$ 30,000,000			$ 900,000,000
Total floating rate debt	505,753,000	505,753,000	1,157,695,000			132,179,000	116,667,000																																													769,813,000	84,580,000	266,994,000	271,215,000	22,000,000
Total mortgages and other loans payable	4,846,731,000	4,846,731,000	4,615,464,000
Preferred units, shares issued														63,900,000
Preferred units (as a percent)				5.00%										5.00%
Preferred units, liquidation preference (in dollars per share)														$ 1.00
Preferred units, shares redeemed														63,800,000
Controlling interest in the joint venture (as a percent)																																											51.00%
Repaid loan																																																	775,000,000	447,200,000
Refinanced mortgage																																																	900,000,000	775,000,000
Term																																																	12 years	7 years
Interest rate added to base rate (as a percent)					3.00%																																													2.85%
Interest rate, description					1-month LIBOR																																													90-day LIBOR
Variable interest rate floor (as a percent)																																																		3.60%
Loss on early extinguishment of debt	10,000	18,523,000																																															18,500,000
Prepayment penalty																																																	7,600,000
Maximum facility capacity					175,000,000
Extension option available for debt instrument					1 year
Gross book value of the properties collateralizing the mortgages and other loans payable	$ 8,000,000,000	$ 8,000,000,000	$ 7,600,000,000